Porous membrane materials as structured packing for distillation

ABSTRACT

The present invention includes an apparatus and method for use of distillation packing material made from a non-selective meso/microporous membrane that separates light hydrocarbon mixtures.

RELATED APPLICATIONS

This application claims the benefit of provisional application No.60/664,800 filed on Mar. 23, 2005, titled “Porous Membrane Materials asStructured Packing for Distillation”.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to fluid separation anddistillation processes and, more particularly, to the use ofnon-selective meso-micro porous materials (pore sizes ranging from 0.01to a few hundreds micrometer) as structure packing to separate liquidmixtures, including olefins and paraffin.

BACKGROUND OF THE INVENTION

The petroleum industry uses 6.9 quadrillion BTU's of energy per year,40% of this energy is used for distillation nation wide. The energyconsumption used in the distillation process is even larger worldwide.Ethylene and propylene (olefins) are two of the largest commoditychemicals in the U.S. and are major building blocks for thepetrochemicals industry. These olefins are mostly separated by cryogenicdistillation that demands extremely low temperatures and high pressures.Over 75 billion pounds of ethylene and propylene are distilled annuallyin the U.S. at an estimated energy requirement of 400 trillion BTU's.

The largest potential area for energy reduction is in the cryogenicisolation of the product hydrocarbons from the reaction by-products,methane and hydrogen. This separation requires temperatures as low as−150° F. and pressures exceeding 450 psig.

Light hydrocarbon olefin/paraffin separations are dominated by cryogenicdistillation technology at an estimated 1.2×10¹⁴ BTU's expendedannually. In addition, there is enormous capital and operating costsassociated distillation. This has motivated an appreciable amount ofeffort towards pursuing alternative olefin/paraffin separationtechnologies. In the past decade, reactive or selective membranes forthe olefin and paraffin separation had been widely investigated.However, the difficulty of long term stability associated with thesefacilitated transport membranes is a major obstacle.

Recently, the possibility of capillary condensation using a porousstructure to separate light gases was explored, and the potential ofusing non-selective membrane for the olefin/paraffin separation wasshown (see U.S. Pat. No. 6,039,792). In 2003, work was reported on usingnon-selective and non-porous membrane as structured packing to replacethe distillation column for water-isopropanol separation and alsosuggested the possibility of using this technology for light hydrocarbonmixture separations (reference: Zhang, G. et al., “Hollow fibers asstructured distillation packing,” Journal of Membrane Science 2003, 215,185-193).

Non-selective micro-porous membranes have been used as a barriermaterial in membrane contactors for vapor/liquid or liquid/liquid masstransfer, desalination, concentrating fruit juice and enriching theoxygen in blood during open-heart surgery (reference: “Hollow fibermembrane contactors”, Journal of Membrane Science, 159 (1999) 61-106).

A unique feature of micro-porous membranes is a large surface areawithin a small volume, normally more than 3000 m²/m³, that provides anincreased rate of mass transfer at least 10-20 times faster than theconventional tray and structured packing materials (creating an intimatecontact between liquid and vapor phases). Although the high efficientSulzer Chemtech® structured packing materials (AG Corporation), such as250 Y/X, have conquered the chemical industry globally in the pasttwenty years, it has not been used in the C₃ and C₄ splitters since theyrequire high liquid loads where structured packings typicallydeteriorate in performance. An advantage of using hollow fiber asstructured packing is that the liquid and vapor velocity are notinterrupted by the two-phase fluid mechanics. Therefore, the columnpacked with hollow fibers can operate above the normal flooding limitsand below the normal loading limits.

However, the general requirement for use of a porous membrane inmembrane distillation is that the membrane should not be wetted by theprocess liquids, because it is believed that the membrane wall oncefilled with liquid will increase the mass transfer resistance betweenthe liquid and vapor phase, as well as reduce the flux of process fluidpassed through the membrane wall. Therefore, historically it has been arequirement that the membrane used in the membrane distillation exhibitnon-wetting characteristics (reference: “Membrane distillation”, K. W.Lawson et al., Journal of Membrane Science, 124 (1997) 1-25; “Hollowfiber membrane contactors”, Journal of Membrane Science, 159 (1999)61-106; and, “Designing hollow fiber contactors”, M. C. Yang et al.,AlChE Journal, 32 (1986) 1910-1916).

The present invention includes the uses of a non-selective mesoporousand/or microporous membrane to separate olefinic mixtures from lightvapor byproducts at higher temperatures and lower pressures than arecurrently required.

Various objectives, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectivesand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes anapparatus and method for use of distillation packing material made froma non-selective meso/microporous membrane that separates lighthydrocarbon mixtures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 is a pictorial illustration of a membrane contactor module.

FIG. 2 pictorially shows the concentration profile change across anon-wetting membrane.

FIG. 3 a pictorially illustrates the intimate contact between liquid andvapor within the wall of porous material used as structured packing.

FIG. 3 b pictorially illustrates the concentration profile change acrossa wetting membrane.

FIG. 4 pictorially illustrates parallel porous tubes within adistillation column.

FIG. 5 a is a schematic of a partial reflux distillation setup.

FIG. 5 b is a schematic of a total reflux distillation setup.

FIG. 6 is a schematic detailing liquid and vapor flow paths within atotal reflux embodiment of the membrane distillation setup.

FIG. 7 is a schematic detailing the test setup of the present invention.

FIG. 8 a shows a scanning electron microscope image detailing themorphology of the porous materials used in Module-2.

FIG. 8 b shows a scanning electron microscope image detailing themorphology of the porous materials used in Module-4.

FIG. 9 graphically shows the thermogravimetry analysis (TGA) results ofseveral hollow fiber samples tested in LANL modules.

FIG. 10 graphically shows the swellability test results of hollow fibersamples in hollow fiber modules.

FIG. 11 graphically shows the effect of temperature on the length changeof hollow fiber samples.

FIG. 12 graphically shows flow parameter vs. flow capacity.

FIG. 13 graphically shows the effect of temperature and fluid capacityon the separation efficiency of hollow fiber modules.

FIG. 14 graphically shows the effect of operation temperature on themass transfer time obtained from hollow fiber modules for thepropylene/propane separation.

FIG. 15 graphically shows the column separation efficiency for Modules2, 4, and 6.

DETAILED DESCRIPTION

The present invention includes the use of non-selective mesoporousand/or microporous fibers as structured packing for separation of lighthydrocarbons like olefinic mixtures and olefin and paraffin mixtures.Mesoporous is defined as a material with pore size of less than 0.01micrometer. Microporous is defined as a material with pore size in themicrometer range (a few hundred micrometers to 0.01 micrometer). Testingresults prove that the hollow fiber made of non-selective mesoporousand/or microporous fibers can be used as structured packing, thus,allowing for an alternative technology for conventional distillation.

Non-selective mesoporous and/or microporous materials that may be usedin this application include, but are not limited to: plastics (e.g.polypropylene, polysulfone, polyethylene, polyvinylididene, mixed ester,and polyestersulfone), ceramics, and metals. Preferred embodiments ofthese materials exhibit pore sizes that ranges from about 0.02 to a fewhundred micrometers for fiber wall thicknesses ranging from about 30 to500 micrometers, and, more precisely, when the pore size is less thanabout 0.05 micrometers the corresponding wall thickness should be lessthan about 100 micrometers. When the pore size is larger than a fewmicrometers, the corresponding fiber wall thickness is preferablythicker than 100 micrometers.

The use of hollow fiber material as structured packing overcomesflooding or loading concerns, and offers a wide range of operatingconditions. Furthermore, the large mass transfer area provided by thehollow fibers increases separation efficiency and, therefore, reducesthe operating and capital cost when compared to use of conventionaldistillation packing materials.

Referring to FIG. 1, when a membrane contactor is used for liquid/vaporseparation, fluid #1 flows through the lumen side of the hollow fiber,and fluid #2 flows on the shell side of the hollow fiber. The liquidphase does not penetrate the pores in the membrane. Referring now toFIG. 2, where X, X* are the mole fraction of volatile compounds inliquid phase at operational and equilibrium conditions, and Y, Y* arethe mole fraction of more volatile compounds in vapor phase atoperational and equilibrium conditions, the transport subject to morevolatile species occurs by evaporation from the liquid phase into thepores of membrane at the pore mouth of the liquid side, followed byvapor molecules' diffusion across membrane to the opposite side. In thisscenario, the equilibrium between liquid and vapor phases occurs at thepore mouth, along the membrane surface of the liquid side. The contactarea is limited to the open pores on the surface to where liquidcontacts. Typically, the pore sizes used in this type of membranecontactor ranges from 100 Å to 0.5 μm.

Referring to FIG. 3 a, in the present invention, unlike the methodabove, the micro-porous materials in tube shape, e.g. hollow fiber usedin this work, can be wetted with the process liquid (pores can filledwith liquid (see FIG. 3 b)). By judiciously selecting the morphology ofporous membrane and controlling the pressure difference across themembrane, the penetration rate of liquid can be controlled and thusallow the liquid phase to form an ultra-thin film uniformly coating thecurvature of the large pores (pore size on the vapor side of themembrane can be larger than 50 μm). This surface feature exists withinthe wall of the fiber at the boundary contacting with the vapor phase(see FIG. 3 a).

Therefore, it is preferred that the asymmetric structures with a denserstructure (pore size <0.5 μm) contacts the liquid phase while the opencellular foam structured (pore size ranges from 0.5 to several hundredmicrometers) contacts the vapor phase. The more volatile speciesevaporates into the vapor phase at the boundary at the vapor side whilethe condensed phase flows downward on the membrane surface and/or insidethe wall of the membrane.

Due to the wettibility of the porous materials, the liquid can penetratethrough the entire membrane and thus ensures the full usage of thesurface of the porous materials. Therefore, the mass transfer rate canbe accelerated as the vapor flows through the curvature of the poreswhere the thin liquid film is covered to disturb the boundary layer ofthe vapor phase, thus reduce the resistance of mass transfer process.

A system for separating olefin/paraffin mixtures includes a fluid tightcolumn and a porous material that divides the column into a liquid andvapor chamber. Inside the housing, the vapor and liquidcounter-currently flow through both sides of the boundary materials thatcan be wetted with the process liquid. The vapor and liquid phasesinteract with each other either at the membrane surface contacting withvapor phase and/or inside the wall of the porous materials where thehigh boiling point material, e.g. paraffin in the olefin/paraffinmixture is condensed into the liquid phase and purer low boiling pointmaterial, e.g. olefin is left in the vapor phase (see FIG. 3 a).

Although a great effort has been devoted to improve the uniformity ofliquid fed across the column, the thickness of the liquid layer on thesurface of the packing materials is difficult to control. Theappreciable amount of channeling and misdistribution of liquid is alwaysa limiting mass transfer process and thus reduces the separationefficiency of the packing materials. Also, there is not a clear boundarybetween the liquid and vapor phases; for example, even the widely usedSulzer Chemtech® structure packing material 250 Y that provides highseparation efficiency, has not been commercially utilized for propaneand propylene mixture separation due to the difficulty of disengagementof the vapor from the liquid phase.

However, referring now to FIG. 4, by using porous tubes 1 orientated inparallel inside column 5, liquid flows within porous tubes 1 while vaporflows counter currently within channels 2 that are defined by outsidewall 3 of porous tubes 1 and column 5 inner wall 4. The liquid phase isevenly distributed and driven by the gravity flow downward inside theporous tube, the wettibility of the porous materials allows the liquidto drain freely across the membrane while a thin film can be uniformlyformed on the other side of the tube along the entire tube (where theliquid and vapor phases are contacting).

Experimental results show that the largest mass transfer coefficients(>0.01 cm/sec) are obtained with fibers containing large pores (<1 μm-30μm) and exhibiting a thicker wall (up to 450 μm), which is consistentwith the preceding discussion. The height equivalent to theoreticalplate (HETP) using these fibers has been reduced to <20 cm when theliquid flux is up to 200 m³/m³-hr compared to values of >30 cm at theliquid flux less than 100 m³/m³-hr of Sulzer Chemtech® packingmaterials. In this work, the mass transfer surface area is about500-2300 m²/m³. If the mass transfer area is larger than 3000 m²/m³,HETP's less than a few centimeters can obtain.

Since a large specific mass transfer area (a (m²/m³)>1000) is theinherent advantage of hollow fiber configuration, together with theenhanced mass transfer coefficient (K_(G)), the mass transfer time(1/K_(G).a)−reciprocal of the product of mass transfer coefficient andthe specific area can largely be reduced from >50 sec (for conventionalpacking materials) to <1 sec for the current design.

Distillation is the process of heating a liquid solution to drive off avapor and then collecting and condensing the vapor. The separationmechanism is based on the difference in volatility of components in themixture. Referring now to FIG. 5 a, which shows typical distillationcolumn 5, in a typical distillation process, feed solution 10 is fedinto middle section 20 of column 5; high boiling point component 11 isconcentrated and removed from reboiler 25 while low boiling pointcomponent 12 is collected after condenser 30. In order to increase theseparation efficiency, partial amount 13 of low boiling point component12 is returned back to column 5 with remaining distillate (finalproduct) 14 routed to post-processing (not shown).

In evaluating a distillation process, it is convenient to assume acondition of total reflux (all the condensed liquid is returned to thecolumn) as illustrated in FIG. 5 b because this simplifies the operationon a pilot plant (or on a laboratory scale) by limiting continuous feedand withdraw of distillate from the system, thereby reducing thedifficultly of determining the height of a transfer unit (HTU) or HEPTfor the packing materials used in the distillation process.

Referring now to FIG. 6, within hollow fiber distillation column 40,vapor 45 flows up the outside of hollow fiber 41 while liquid 47 flowsdown the inside of hollow fiber 41. The analysis of differentialdistillation is simplified when analyzing total reflux conditions. Theoverall mass balance between the vapor and liquid condensate at anypoint is simply:G=L,   (1)where G and L are the molar vapor and liquid flowrate in column 40. In asteady-state flow condition, Ö, G and L are constant. For the totalreflux 50, Ö called the operation line on a MaCable-Thiele Diagram issimply:x=y,   (2)where x and y are the mole fraction of the more volatile species in thevapor and the liquid phase, respectively. For the mass balance on eithervapor or liquid phase alone a column, the following equations apply:$\begin{matrix}{0 = {{{- G}\frac{\mathbb{d}y}{\mathbb{d}z}} + {K_{y}{a\left( {y^{*} - y} \right)}}}} & (3)\end{matrix}$where K_(y) is the overall vapor phase and liquid phase mass transfercoefficients, respectively, a is the mass transfer area (cm²/cm³), z isthe distance measured from the bottom of the column, y is the molefraction of the most volatile species in the vapor, and y* is the molefraction in equilibrium with the liquid. Thus, the rate equationsderived from the integrated form of Equation 3 is Equation 4:$\begin{matrix}{l = {{\int_{0}^{l}\quad{\mathbb{d}z}} = {\frac{G}{K_{y}a}{\int_{y_{0}}^{y_{l}}\frac{\mathbb{d}y}{y^{*} - y}}}}} & (4)\end{matrix}$Equations (4) may be written as:l=HTU•NTU   (5)where HTU is a height of a transfer unit defined as: $\begin{matrix}{{HTU} = \frac{v_{G}}{K_{G}a}} & (6)\end{matrix}$and NTU is the number of transfer unit given by: $\begin{matrix}{{NTU} = {\int_{y_{0}}^{y_{l}}\frac{\mathbb{d}y}{y^{*} - y}}} & (7)\end{matrix}$

In the petroleum industry, the height equivalent to a theoretical plate(HETP) is commonly used to replace HTU to express mass transfercapability for the structured packing materials in the distillationtower. The HEPT is related to the HTU through a linearization ofoperating and equilibrium curves expression as follows: $\begin{matrix}{{{HETP} = {{HTU}*\frac{L\quad{n\left( {{mG}/L} \right)}}{{{mG}/L} - 1}}},} & (7)\end{matrix}$where m is the slope of equilibrium curve. Since mG/L will changeconsiderably throughout a distillation, the HETP will change as thecomposition changes, even the HTU may not. Therefore, HEPT, combiningmore factors than HTU, is widely accepted by the fractionationresearchers.

Referring to FIG. 7, there were three major components for thedistillation system 100 in which the experiments were carried out:reboiler 110, condenser 120, and membrane module 130. Vapor supplysystem 140 was used to initially charge reboiler 110 with startingmaterials. Five sampling stations 210, 212, 214, 216, and 218, were usedto monitor the vapor and liquid composition at different sections ofsystem 100 during operation. Heater/chiller system 112 was used toprecisely control operating temperature during operation. Thus, bycontrolling the temperature difference between the heater and chiller,the liquid flow rate was controlled. Drain 135 was used to preventliquid buildup on the vapor side. Bypass 115 was used to add liquid toreservoir 125 in preparation for initial startup.

View port 170, mounted between condenser 120 and the top of membranemodule 130 was used to ensure that hollow fibers 40 were filled withliquid during the experiment. A rheonik coriolis meter 200 (Liquidcontrols, LLC), with RHM 03 sensor and RHE08-transmitter, was used tomeasure the liquid flow. As the system was working in a total refluxcondition, the vapor flow rate was back calculated from the liquid flowbased on the mass balance between the vapor and liquid flow.

During the experimental run, the vapor and liquid samples (˜10 ml and 40Psig) were collected into a sample bottle at the five sample stations.HP M series micro gas chromatograph (GC) (Model number G2762A) was usedto characterize the vapor compositions. Two columns were dedicated todifferent gases: column A, MS-5A 10 m, was used for compressed gasessuch as He, H₂ and light vapor molecules and column B was used forcompressed gases, such as CO₂, N₂ and light hydrocarbons.

The hollow fiber modules were fabricated by potting the fibers insidepolycarbonate tubes with an active length of 36.8 cm. The innerdiameters of the tubes were about 0.5 inch (1.27 cm). The detailed fiberand module parameters are summarized in Table 1. Five types of fiberswith the pore size ranging from 0.04 to 30 μm, and the wall thicknessfrom 30 to 440 μm were selected for the evaluation tests. Module fibersused in this experiment were procured from Setec, Inc. (Module-2), Pallcompany (Module-3), Spectrumlab (Module-4 and Module-9), and Celgard®fiber company (Module-6).

The mass transfer area per unit volume ranged from 1094 to 1473 m²/m³,which was calculated based on the outside surface area of hollow fibers.Comparing to Sulzer chemtech® 250 and 500 structured packing materials,the mass transfer area is about 4 to 6 times larger. The packing densityranged from 11-45%. Note that the across area ratio (A_(g)/A_(L)) ofvapor to liquid defines the open area between vapor and liquid to flowwithin column.

Five trials, and, thus, five differing hollow fiber modules were testedunder total reflux conditions, were conducted using the setup in FIG. 7.An olefinic/paraffinic mixture of ˜30% propane and 70% propylene molepercent was used as the starting material. During the experiments, theconcentration of propane and propylene changes within 10%, thus theslope of their equilibrium line is almost constant. Therefore, the NTUwas determined from the measured propylene compositions at the top andthe bottom of the column. TABLE 1 A summary of module hollow fiberparameters Module-2 Module-3 Module-4 Module-6 Module-9 FiberPolypropylene Polyvinylididene Polysulfone Polypropylene Mixed Estermaterial Fluoride Pore size 0.1-30 <0.02 0.05-0.5 0.04 0.1-1.0 (μm)Fiber ID 1.74 0.626 0.480 0.240 0.680 (mm) Fiber OD 2.62 1.20 0.6300.300 0.850 (mm) Thickness 440 288 75 30 85 (μm) Number of 19 20 70 19847 fibers Packing 45 17 17 11 22 density (%) Packing 2287 327.7 587 637873 factor (ft⁻¹) A_(g)/A_(L) 0.55 17.1 8.28 12.56 5.64 a 12.35 5.9510.94 14.73 13.39 (cm²/cm³)

FIGS. 8 a and 8 b show the scanning electron microscope (SEM) images ofthe polypropylene fiber used in Module-2 module and polysulfone fiberused in Module-4 module, respectively.

A thermogravimetry analysis (TGA) was used to test the stability of thehollow fiber samples in all the modules. The TGA results for these agedfiber samples are shown in FIG. 9. The TGA results show that the weightpercent (W%) change over time is almost identical for each fiber sample,concluding that all of these fibers samples are stable after they weresoaked in pentane at room temperature for one year. Several hollow fibersamples were tested in olefin and paraffin mixture for several monthswith the corresponding TGA results essentially the same as the resultsobtained from samples soaked in pentane, indicating that pentane can beused as surrogate solvent to test the hollow fibers.

The material characteristic of minimum swellability is preferred infiber material in order to minimize geometry change during operation.Thus, the swellability of the fiber modules has been tested in severalorganic solvents as well. The tested results, shown in FIG. 10, indicatethe swellability of these fibers was less than 10% in all testedsolvents. Based on observation of liquid flow down the fibers whileusing pentane as the solvent, it is apparent that all of the fibers areeasily wetted with light hydrocarbon materials.

Referring to FIG. 11, once the hollow fibers were soaked in pentane, thetemperature effect on the change in length of the hollow fibers, and,therefore on separation performance was negligible. All of these resultsprove that the thermal stability of these hollow fibers is suitable tothe distillation application for organic solvents

FIG. 12 is a plot of flow parameter vs flow capacity for the testedmodules shown in Table 1. The plot demonstrates that a column (ormodule) including porous tubes in hollow fiber form can handle a widerange of flow conditions. These flow conditions are at least 100 timeslarger than the flooding limit of conventional packing materials.Further, the tested modules operated below the loading line ofconventional packing materials. This proves that the tube-shellconfiguration of porous materials in a distillation column can provideseparate channels for vapor and liquid to prevent flooding and loadingproblems, and at the same time provide a preferred large contact areabetween the vapor and liquid.

The separation performance of these modules is summarized in Table 2.The HETP value determined from the Module-2 module was 3.0 inch (7.6cm), indicating a mass transfer time as low as ˜0.40 sec, which was thebest mass transfer rate of all the modules tested. Thus, these resultsprove that hollow fiber materials can be used as structured packingmaterials in olefin/paraffin distillation separation. It is worthy tonote that among these modules, Module-3 has the worst performance(meaning larger mass transfer time). This poor performance is due to thefibers packed in Module-3 having much smaller pore sizes (0.02 μm) and alarge wall thickness (˜288 μm).

The dense morphology creates a greater resistance layer between liquidand vapor, thereby reducing both the heat and mass transfer rate betweenthe two phases. This observation again proves that the large and/or openporous structure inside the membrane wall is critical to allow theprocess liquid to easily flow across the membrane under a small drivingforce (<0.5 psi) and to interact with the vapor phase promoting heat andmass transfer.

Note that the comparatively poor separation performance of Module-3still provided a mass transfer time of less than 40 seconds. Whereas,for the Sulzer Chemtech® series packing materials, the mass transfertime is greater than 40 seconds (reference: “Non-selective membrane forseparations”, E. L. Cussler, Journal of Chem. Tech. and Biotech, 78,98-102). TABLE 2 Summary of separation performance of tested modulesModule-2 Module-3 Module-4 Module-6 Module-9 Module temp 24.3 21.1 17.619.7 17.5 (° C.) System 151.3 140.4 124.9 131.7 124.0 pressure (Psig)Liquid 8.0 4.67 4.12 11.2 11.2 flowrate (g/min) Vapor 22.26 3.36 3.368.02 9.87 velocity V_(g) (cm/sec) C₃ ⁼ (Top of 86.0 72.6 75.1 76.9 75.0module) C₃ ⁼ (Bottom 79.0 71.3 71.7 70.4 70.0 of module) NTU 4.5 0.651.53 2.63 2.00 HTU (cm) 8.2 56.6 24.2 13.9 18.4 HEPT (cm) 10.5 68.8 29.416.9 22.4 Mass 0.224 0.010 0.013 0.041 0.040 transfer coeff - K_(G)(cm/sec) Mass 12.35 5.95 10.94 14.73 13.39 transfer area (a) (cm²/cm³)Mass 0.37 16.8 7.18 1.73 1.86 transfer time 1/(K_(G) · a) (sec)

In distillation processes, as the temperature increases, the relativevolatility increases favoring separation. However, the density ratiobetween the liquid and vapor for propane and propylene increase from 22to 46 as the temperature decreases from 25 to 0° C., and sodisengagement of liquid and vapor becomes easier in the lowertemperature range.

In order to explore the effect of temperature on the separationperformance, a series of experiments was conducted over a temperaturerange from 0 to 25° C. for the previously mentioned modules. Referringto FIG. 13, the experimental results proved that higher separationefficiency was achieved at lower temperatures. The HETP remains low (<40cm) for a large liquid flux range (>200 cm³/cm³-sec) for both Module-4and Module-6. Note that when the operation temperature was less than 6°C., the mass transfer time (1/K_(G).a) was typically less than 10seconds, which is at least 5 times better than the conventional packingmaterials (see FIG. 14).

Historically, the separation of propylene and propane has been adifficult separation for the petrochemical industry. This is due in partto the narrow boiling point difference between propylene and propane atambient conditions, which is only 7.5° C. The relative volatility isless than 1.2 when the temperature is close to 0° C. A 1° C. temperaturedifference along the column for a propylene/propane (70/30) mixture canresult in ˜15% propylene concentration difference. However, due to thelargely open structure in the conventional packing materials, convectiveheat transfer between liquid and vapor is large, making it difficult tomaintain a couple of degrees temperature gradient within a shortdistance (<50 cm).

On the contrary, due to the confined diameter of the hollow fibersinside dimension and meso/ microporous structure on the wall, the liquidflow is typically in the laminar flow region. Thus, the convection amongthe fluids is largely decreased making larger temperature gradientswithin short distances possible. Thus, a small HETP (or HTU) isexpected. The thermodynamic efficiency is typically larger than 85% whenthe modules are operated at their optimized operation zone (see FIG.15).

Theoretically, when the liquid flow slows down, convection between thetwo phases is reduced and high separation efficiency is expected.However, at very low flows, an uneven liquid film on the surface of thepacking materials can occur. This can be the result of insufficientliquid to cover the surface area of the packing material or channelingof liquid flow. Thus, the reduced liquid surface area results in lessefficient mass transfer.

However, with the present invention, utilization of porous tubematerials, such as mesoporous and microporous hollow fibers, provides apore size on the liquid side of wall that is less than a fewmicrometers, but includes a wall thickness with the same order ofmagnitude as the tubes inside diameter (from about ten micrometers to afew hundred micrometers) (see FIG. 8 a). Thus, the liquid in the liquidphase is uniformly distributed into the tube while the liquid thin filmforms on the wall of the pores and/or the outer surface.

Due to a much thinner and uniform film formed on the surface of theporous materials, the total amount of liquid required to cover thesurface of porous materials is much less than that required to cover thesurface of conventional packing materials. Therefore, when the liquidloading drops below the loading line (dashed line) in FIG. 12, themodules were able to maintain their separation efficiency.

The present invention provides more surface area and a more uniformdistribution of liquid per unit volume of material than conventionalmaterials, and, thus, correspondingly yields enhanced mass transfer perunit of volume. For example, referring to FIG. 13 for Module-2, when theliquid flow is below 2.5 g/min (<0.1 cm³/cm^(2.)sec), the HETP is stillbelow 15 cm (<6 inch). These results cannot be achieved through the useof conventional packing materials.

In summary, the present invention is a porous tube material that may beused as structured packing in a distillation process that provides alarger process flow rate operating range than current conventionalpacking materials. The present invention provides for an HETP of lessthan 10 cm and mass transfer time less than 1 second. A wide range ofmaterials may be used as the porous tube material, to include ceramics,metals, and polymers. Therefore, the operational ranges of distillationssystems using these materials can cover temperatures from about −60° C.to 200° C., and pressures from a few psig to several hundred psig. Thus,any type of distillation process within these parameters may beperformed with the present invention, to include separation of lighthydrocarbons such as olefin and paraffin.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A distillation packing material for separating light hydrocarbonmixtures comprising a non-selective meso/ microporous membrane.
 2. Thedistillation packing material of claim 1 bundled in a parallel tubeconfiguration.
 3. The distillation packing material of claim 1 wheresaid distillation packing material density ranges from about 5% to 45%.4. The distillation packing material of claim 1 where said distillationpacking material is selected from the group consisting of:polypropylene, polysulfone, polyethylene, polyvinylididene, mixed ester,and polyestersulfone.
 5. The distillation packing material of claim 1where said distillation packing material is a ceramic.
 6. Thedistillation packing material of claim 1 where said distillation packingmaterial is a metal.
 7. A distillation apparatus for separating lighthydrocarbon mixtures, comprising: a. a distillation packing materialcomprising a non-selective meso/ micro porous membrane bundled in aparallel tube configuration, b. a distillation column, defining a topand a bottom, packed with said non-selective meso/ micro-porous membranedefining a liquid side and a vapor side within said distillation column,c. a reboiler used to heat liquid light hydrocarbon mixtures creating avapor on said vapor side and providing a thermal driving force withinsaid distillation apparatus, and d. a condenser used to condense saidvapor creating a liquid on said liquid side.
 8. The apparatus of claim7, further comprising a drain on said vapor side of said distillationcolumn to drain said liquid out of said vapor side.
 9. The apparatus ofclaim 7, where said distillation packing material density ranges fromabout 5% to 45%.
 10. The apparatus of claim 7, further comprising abypass on said vapor side for accumulating said liquid at said top toprime said distillation column before said vapor is introduced into saiddistillation column, thereby providing liquid flow to start theseparation process.
 11. The apparatus of claim 7, further comprising areservoir operatively connected to said top of said distillation columnfor holding said liquid inventory before and during said distillationprocess.
 12. The apparatus of claim 7, where an across area ratio ofsaid vapor to said liquid ranges from about 0.5 to
 20. 13. The apparatusof claim 7, where said distillation packing material is selected fromthe group consisting of: polypropylene, polysulfone, polyethylene,polyvinylididene, mixed ester, and polyestersulfone.
 14. The apparatusof claim 7, where said distillation packing material is a ceramic. 15.The apparatus of claim 7, where said distillation packing material is ametal.
 16. A method for separating light hydrocarbon mixtures,comprising: a. providing a distillation column packed with anon-selective meso/microporous membrane bundled in a parallel tubeconfiguration that defines a liquid side and a vapor side, b.introducing said light hydrocarbon mixture to said liquid side of saiddistillation column, and c. separating said light hydrocarbon mixture.17. The method of claim 16 further comprising the step of returning apartial amount of a low boiling point component of said lighthydrocarbon mixture back to said distillation column to increaseseparation efficiency.